Shell script termination with exit function in backquotes

Maxim Khitrov max at
Sat Mar 19 16:15:58 UTC 2011

On Mon, Mar 14, 2011 at 6:40 PM, Andres Perera <andres.p at> wrote:
> On Mon, Mar 14, 2011 at 7:46 AM, Maxim Khitrov <max at> wrote:
>> On Mon, Mar 14, 2011 at 3:16 AM, Andres Perera <andres.p at> wrote:
>>> On Sun, Mar 13, 2011 at 9:49 PM, Devin Teske <dteske at> wrote:
>>>> If you make the changes that I've suggested, you'll have consistent execution. The reason you're having inconsistent behavior is because Linux has /bin/sh symbolically linked to /bin/bash while FreeBSD has a more traditional shell (we'll call it bourne shell "plus").
>>> that is misleading because command substitutions have traditionally
>>> invoked subshells, and freebsd sh(1)/ash is an exception, not the norm
>>> in this case, ksh and bash deviates are clearly closer to standard
>>> bourne behaviour
>> Thanks for that explanation. I can understand the benefits of
>> optimizing away subshell execution, but that can clearly lead to
>> unexpected behavior. Is there some documentation on when this
>> optimization is utilized (i.e. the command executed without a
>> subshell)? Would I be correct in assuming that it is only restricted
>> to built-in commands that are known not to produce any output, such as
>> 'exit'?
> i would check the source, autoconf docs, and
> netbsd has  been patched to fix `exit 1`, according to the last site

Here's another, but related, problem that I just ran into. The man page reads:

     Commands may be grouped by writing either
           { list; }
     The first form executes the commands in a subshell.  Note that built-in
     commands thus executed do not affect the current shell...

Here's my script:


{ A=1; };             echo $A
echo | { B=2; };      echo $B
{ C=3; } > /dev/null; echo $C

And here's the output:



Where did the '2' go? Again, I have to assume that when stdin is piped
to a group of commands, those commands are executed in a subshell
despite curly braces. But where is this behavior documented? It seems
that there are a lot of corner cases that can only be understood if
you are familiar with the shell implementation. Documentation can
certainly be improved in places.

- Max

More information about the freebsd-questions mailing list